A quantitative version of the Gibbard-Satterthwaite theorem
Seminar Room 1, Newton Institute
AbstractConsider an election between q alternatives, where each of the voters rank the different alternatives, and the winner is determined according to some predefined function of this voting profile. Such a social choice function is called manipulable, if a situation might occur where a voter who knows the rankings given by other voters can change her ranking in a way that does not reflect her true preference, but which leads to an outcome that is more desirable to her. Gibbard and Satterthwaite proved that any social choice function where more than two alternatives can be selected is manipulable, unless it is a dictatorship (where the outcome of the election only depends on the choices of one voter). In the case where the social choice function is neutral, namely when it is invariant under changing the names of the alternatives, we prove a lower bound on the fraction of manipulable preference profiles which is inverse polynomial in the number of voters and alternatives. Our proof in fact does not rely on discrete harmonic analysis - finding an analytic version of the proof would be the role of the audience. Joint work with Marcus Isaksson and Elchanan Mossel.
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